[ 
  { 
    'supplier_id' => 17,
    'item_foo'    => 3,
    'item_bar'    => 7
  },
  {
    'supplier_id' => 28,
    'item_foo'    => 5,
    'item_qux'    => 202
  }
]

##</code><code>##

{
  '28' => {
            'item_foo' => 5,
            'item_qux' => 202
          },
  '17' => {
            'item_foo' => 3,
            'item_bar' => 7
          }
};

##</code><code>##

sub set_stock
{
    my( $self, $supplier, $line, $stockLevel ) = @_;

    my $supplierList = ();
    my $supplierNumber = $supplier->supplier_id; 

    # $supplierList will be a reference to a hash of references to hashes.
    unless( $supplierList = $self->get('supplier_list') ) 
    {
        die( "Cannot set stock level when supplier list is corrupt: $supplierList" );
    }

    my $reference = $line->reference;
    $supp->{ $supplierNumber }{ $reference } = $stockLevel;
}